Today, IP Multimedia Subsystem, IMS, is an established technology within data and telecommunications. IMS can be described as an architectural framework for delivering internet protocol (IP) multimedia to mobile users. It was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), and is part of the vision for evolving mobile networks beyond GSM. Its original formulation (3GPP R5) represented an approach to delivering “Internet services” over GPRS. This vision was later updated by 3GPP, 3GPP2 and TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed line. To facilitate the integration with the Internet, IMS as far as possible uses IETF (i.e. Internet) protocols such as Session Initiation Protocol (SIP). According to the 3GPP, IMS is not intended to standardize applications itself but to aid the access of multimedia and voice applications across wireless and wireline terminals. This is done by having a horizontal control layer that isolates the access network from the service layer. Services do not need to have their own control functions, as the control layer is a common horizontal layer. Since IMS was conceived years ago, it is becoming increasingly easier to access content and contacts using mechanisms outside the control of traditional wireless/fixed operators.
IMS requires IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU). Both are not phone numbers or other series of digits, but Uniform Resource Identifier (URIs), that can be digits (a tel-uri, like tel:+1-555-123-4567) or alphanumeric identifiers (a sip-uri, like sip:john.doe@example.com). There can be multiple IMPU per IMPI (often a tel-uri and a sip-uri). The IMPU can also be shared with another communication terminal, so several communication terminals can be reached with the same identity, for example, a single phone-number for an entire family.
Session Initiation Protocol (SIP) is an application-layer control (signalling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. SIP runs on top of several different transport protocols.
The Session Initiation Protocol, SIP, is disclosed in detail in the paper “SIP: Session Initiation Protocol”, by J. Rosenberg et. al., RFC 3261, The Internet Society, June 2002.
SIP is also discussed in the paper “The Reason Header Field for the Session Initiation Protocol (SIP)”, by H. Schulzrinne et. al., RFC 3326, The Internet Society, December 2002. This paper discusses a further use of the Reason Header field.
A proxy server can send a session invitation to a number of locations at the same time. This type of parallel search is known as forking. This is for example the case when several users share a common IP Multimedia Public Identity, IMPU. However, there are problems in connection with this forking process. When several users share a common IMPU and a session invitation is directed to the common IMPU, and one of the users accept the invitation, the other users receive no information about the outcome of the session invitation. They do not know if the originator has cancelled the session invitation or if a user associated with the common IMPU has accepted to the invitation.